Many technical applications comprise some form of antenna function, in which signals are received or transmitted via air. Examples of such applications are radio devices, TV sets, mobile telephony systems and radar systems.
The demands of directional function of an antenna varies with the application. A radio device shall be able to receive signals from different radio stations independent of where it is placed and therefore, an antenna of the radio device shall be sensitive in all directions in the horizontal plane. However, a TV receiver shall only be sensitive for signals coming from the nearest TV antenna. An antenna of a TV receiver has to be arranged so it is sensitive in particular for signals coming from the nearest TV antenna. Thus, an antenna of a TV receiver has to be arranged so it is sensitive in particular for signals coming from a particular direction, and signals coming from other directions have to be reduced as much as possible. An antenna in a base station in a mobile telephony system shall be able to receive signals from mobile telephones independently of where they are situated. When the base station shall transmit to one of the mobile telephones it can be desirable to transmit signals only in one direction to the particular mobile telephone, to be able to save power and not to interfere with other radio communication.
A type of antenna which can have directional function is the so called group antenna. A group antenna comprises a number of radiation elements, co-operating to give desirable radiation characteristic. The radiation elements consist normally of dipoles, horns or micro-strip elements (so called patches). Furthermore, an active group antenna comprises a number of transmitters, via a distribution network receiving an input signal and transmitting output signals. The output signals are employed to feed the radiation elements. The distribution network together with the transmitters constitutes a lobe-forming network. The group antenna is constructed so the output signals transfer to the radiation elements directly or via some form of external lobe-forming network, e.g. a Butler matrix or a Blass matrix. Controlling of the directional function of a group antenna--an antenna diagram--is obtained by controlling mutually phase-shifting and amplification of the transmitters. The group antenna can of course also be used for reception. Then, the group antenna comprises a number of receivers, wherein the group antenna is so constructed that signals received by the radiation elements are transferred to the receivers, e.g. via the external lobe-forming network.
To be able to achieve a particular antenna diagram of a group antenna with relatively good accuracy it is required that the amplification of the transmitters and in particular the phase-shifting can be well controlled. To be able to obtain this the transmitters must be calibrated. Prior art discloses a large number of proposals of constructions and methods for calibration of group antennas. Often the calibration is done in calibration sets where special calibration signals are employed. For instance, U.S. Pat. No. 5,063,529 describes how a group antenna can be calibrated for reception employing special calibration signals.
A disadvantage with calibrating during calibration sets by employing special calibration signals is that the normal operation of the group antenna must be interrupted during the calibration. For instance in a mobile telephone system this causes that part of the time cannot be used for the normal communication, of course implying decreased incomes for an operator.